1. Field of the Invention
The present invention generally relates to an image forming apparatus, and more particularly to a pancake window display system employing one or more switchable holographic elements.
2. Description of the Relevant Art
Pancake window display systems (often times referred to as in-line infinity display systems) are capable of forming images at or closer than infinity of an object or a plurality of optically superimposed objects. Pancake window display systems find application in aircraft simulators, spacecraft simulators, or in head mounted displays for use in, for example, virtual imaging. Pancake window display systems, so called because of their minimal depth, represent a major achievement in terms of simultaneously maximizing field of view, eye relief, and image quality in a compact and lightweight configuration.
FIG. 18 shows one embodiment of a prior art pancake window display system 310 that includes a first linear polarizer 312, a curved, spherical beam splitting mirror 314, a first quarter wave plate 316, a beam splitting mirror 318, a second quarter wave plate 320, and a second linear polarizer 322. In operation, first linear polarizer 312 imposes linear polarization on light from a source (not shown in FIG. 18) passing therethrough. The direction of the polarization of first linear polarizer 312 is represented by vertical arrow 324. Linearly polarized light encounters first quarter wave plate 316 after being transmitted through partially transmitting, spherical beam splitting mirror 314. The first quarter wave plate imparts a circular polarization to light passing therethrough. Right curved arrow 326 shows that light emerging from first quarter wave plate 316 is right circularly polarized. This right circularly polarized light next encounters partially transmitting, partially reflecting beam splitting mirror 318, a fraction of which passes therethough to encounter second quarter wave plate 320. Second quarter wave plate 320 acts to change the write circularly polarized light transmitted through mirror 318 back to linearly polarized light having a direction of polarization oriented at 90xc2x0 to the direction of polarization 324 of the first linear polarizer 312. This is indicated in FIG. 18 by horizontal arrow 330. The linear polarized light emitted from second quarter wave plate 320 is blocked at second linear polarizer 322 which has a direction of polarization parallel to that of first linear polarizer 312.
The fraction of right circularly polarized light from first quarter wave plate 316 which is reflected at beam splitting mirror 318 is converted by such reflection into circularly polarized light of the opposite rotation, i.e., into left circularly polarized light in the case assumed. This is indicated in FIG. 18 by left curved arrow 332. In its reflective passage back towards first linear polarizer 312, this left circularly polarized light again encounters first quarter wave plate 316 which transforms the left circularly polarized light into linearly polarized light with a direction of polarization perpendicular with respect to the direction of polarization provided by first linear polarizer 312 as represented by horizontal arrow 334. A portion of this linearly polarized light is reflected and collimated by spherical beam splitting mirror 314 without change in the orientation of its polarization direction. The light so reflected and collimated becomes left circularly polarized after passing through first quarter wave plate 316 as indicated by left curve arrow 336. A fraction of this light is then transmitted through beam splitting mirror 318 and converted by second quarter wave plate 320 into linearly polarized light having a polarization direction parallel to the polarization direction of the first linear polarizer as indicated by arrow 340. This light, accordingly, is permitted to pass through second linear polarizer 322 and constitutes only a fraction of the unpolarized light from the image source (not shown in FIG. 18) that is visible to an observer 342.
The arrangement of the pancake window display system 310 shown in FIG. 18 obviates the use of an oblique beam splitting mirror across the axis of the spherical mirror 314 so that the optical elements can be assembled into a compact package. With the exception of the curved mirror 314, all the elements of pancake window display system 310 are in the form of flat sheets, thereby imparting a relatively thin cross section. However, the curved mirror by its nature cannot be reduced to flat sheet form.
FIG. 19 shows a second prior art pancake window display system 350 which includes all of the elements 312-322 of pancake window display system 310 shown in FIG. 18 except for curved, spherical beam splitting mirror 314. Pancake window display system 350 of FIG. 19 employs a static, reflective type holographic analog 352 of a curved mirror in place of the conventional curved mirror 314 of FIG. 18. Typically, such analog 352 is formed by superimposing a coherent monochromatic xe2x80x9creferencexe2x80x9d beam of light and a coherent xe2x80x9cobjectxe2x80x9d beam of light upon a transparent photo sensitive layer to form an interference pattern within the photosensitive layer. The layer is then photographically developed to produce the holographic analog.
As shown in FIG. 19, pancake window display system 350 operates in a manner substantially similar to the pancake window display system 310 shown in FIG. 18. Advantageously, all elements of pancake window display 350 are reduced to a flat sheet form that reduces a longitudinal thickness and weight substantially. However, while pancake window display system 310 shown in FIG. 18 provides a broad band, or colored image to viewer 342, pancake window display system 350 shown in FIG. 19 can provide only a narrow band or monochromatic image to viewer 342 as a result of employing the static, reflective type holographic analog 352.
The present invention provides one or more dynamic, switchable holographic elements which can be used in a pancake window display system. Each of the one or more switchable holographic elements is configured to operate between active and inactive modes. In the active mode, the switchable holographic element substantially alters a substantial portion of light incident thereon. In one embodiment, the switchable holographic element operating in the active state, reflects and collimates a substantial portion of light incident thereon. In this embodiment, the switchable holographic element in the active state defines a holographic analog of a concave, spherical mirror. In the inactive state, the switchable holographic element transmits substantially all light incident thereon without substantial alteration. In one embodiment, the holographic analog of the concave, spherical mirror of the switchable holographic element is erased.
In another embodiment, the present invention provides three, dynamic, switchable holographic elements, each one of which operates between the active and inactive states in accordance with signals provided by a logic control circuit. Each of the three switchable holographic elements operating in the active state defines a holographic analog of a concave, spherical mirror which reflects and collimates a select bandwidth of light incident thereon. In the inactive state, each of the three switchable holographic elements erases its holographic analog of the concave spherical mirror so that substantially all light incident thereon is transmitted thereto substantially unaltered. In one embodiment when active, the first of the three holographic optical elements is configured to reflect and collimate narrow band red light, the second holographic optical element is configured to reflect and collimate narrow band blue light, and the third holographic optical elements is configured to reflect and collimate narrow band green light. In this embodiment, the control logic circuit, coupled to each of the three holographic optical elements, sequentially activates and deactivates the three optical elements one at a time so that the three switchable optical elements can reflect and collimate red, blue and green image light sequentially.